JP3236285B2 - High-speed AGC circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an AGC (Automatic Gain Control) system for maintaining constant signal levels of bursts having different signal levels in high-speed burst transmission. For example, the present invention relates to an AGC circuit for stably receiving an optical signal even when there is a difference in received light level in economical communication using a star coupler in optical communication using an optical fiber.

(2) Prior Art Before describing a conventional AGC (automatic gain control) circuit, a situation in which burst signals having different signal levels occur in communication in high-speed burst transmission will be described.
FIG. 1 shows an example of a communication system using an optical fiber. 1
Is an optical fiber transmission line, 2 is an optical transmitter, and 3 is an optical receiver. 4 is a star coupler (SC). In this configuration, a star coupler (SC) 4 is connected to an optical fiber transmission line to accommodate a plurality of subscribers (users). In the figure, only the subscriber #j is shown. Here, #j means a number, that is, No. This configuration is economical because one station device (optical receiver) is shared by a plurality of subscribers. Further, the optical fiber transmission line 1 having a length La between the station (optical receiver side) and the star coupler 4 can be shared by a plurality of subscribers. In this configuration, a plurality of subscribers can be accommodated economically. 1: 1 (ponit-
Even in to-point optical communication, the loss of the optical fiber transmission line changes depending on environmental conditions such as vibration. However, in the configuration of FIG. 1, the distance Lb between the star coupler (SC) 4 and the subscriber (on the optical transmitter side) varies, the loss of each output port of the star coupler 4 varies, and the optical transmitter 2 Due to the transmission light level difference, the loss value will vary greatly. This may result in a light level difference of 10 dB or more. In the configuration shown in FIG. 1, since a transmission line is shared by a plurality of subscribers, when the subscribers communicate with the same wavelength, the transmission line is used in a time-division manner.

FIG. 2 shows the time positional relationship of the signals of each subscriber. Each subscriber transmits a signal in a predetermined time slot. G in the figure is a guard time, which is provided in order to absorb variations in the distance Lb (FIG. 1) between the star coupler and the subscriber and prevent information of each subscriber from overlapping. Since the transmission path is shared by each subscriber, the information transmission rate of the transmission path is N times or more (N: the number of accommodated subscribers) of the information transmission rate B (b / s) of each subscriber, which is extremely high. .

Conventionally, in order to absorb such changes and variations in optical loss and perform stable reception, the gain has been conventionally changed by changing the gain of the optical receiving circuit. This so-called AGC (automatic g
Ain control) is a technique for detecting the level of received light and changing the amplification degree of the light receiving circuit based on the value. This technique is widely used, but has the following problems when applied to an optical fiber transmission line as shown in FIG. That is, in the case of the configuration shown in FIG. 1, information from each subscriber may be transmitted at high speed and as burst information having significantly different optical levels, as shown in FIG. 3 (a). For example, B
When N = 16 at −1.5 Mb / s, the transmission speed of the optical fiber transmission line is 24 Mb / s or more. The length of each time slot
If the time is set to 125 μs, an AGC that responds at a very high speed of 40 to 100 nsec is required for stable reception within the first few bits of each time slot. In the conventional AGC method,
The level of the received light is detected, and the amplification degree of the light receiving circuit is changed based on the detected value.

FIG. 4 shows a circuit example based on the AGC system of the prior art. Reference numeral 5 denotes a variable gain amplifier whose amplification degree (gain) changes according to a voltage applied to a terminal g in the figure. 6 is a low-pass filter, and 7 is a rectifier. The rectifier 7 and the low-pass filter 6 detect the peak level of the signal, and change the gain of the amplifier 5 so that the signal level becomes a constant output.

(3) Problems to be Solved by the Invention In order to set a high-speed response AGC within the first few bits of each time slot in this configuration, a loop composed of the amplifier 5, the rectifier 7, and the low-pass filter 6 needs to operate at high speed. It is necessary to respond. Therefore, it is necessary to set the time constant of the filter to a short time of about several bits. By designing in this way, a high-speed response can be realized even in the conventional method. However, even when the input signal is continuously “0” (here, zero is associated with the level low “L”), the filter can be used. Has a shortcoming that the control voltage of the amplifier 5 also changes, resulting in an excessively high output level with respect to the immediately succeeding signal "1"("H"). This is because if the variable gain amplifier 5 outputs a constant level when "0" continues, the gain is set to the maximum or near the maximum, and the output becomes excessive when "1" is input. is there. As a result, the output level fluctuates greatly depending on the input signal pattern (that is, the pattern in which “0” and “1” occur). Therefore, if the conventional method is designed to enable high-speed response, the AGC function that operates stably cannot be achieved.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an AGC circuit for configuring a stable receiver by making signal levels of bursts having different signal levels constant in high-speed burst transmission.

(4) Means for Solving the Problems In order to achieve this object, a high-speed AGC circuit according to the present invention is an AGC circuit for making the signal level of bursts having different signal levels in high-speed burst transmission constant.
A variable gain element, a level detection circuit, a capacitor, and a switch, wherein the variable gain element is connected so that a gain changes according to a terminal voltage of the capacitor, and is further arranged to detect an output signal level of the variable gain element. has been the level detecting circuit constant T ₂ time for discharging the constants T ₁ and the capacitor when for charging the capacitor at the detected signal level is, satisfy the relationship of T ₁ << T _2, and the The switch is forcibly discharged and reset by the switch using a control signal obtained from the frame timing information of the burst in an empty time slot of the burst.

(Operation)

FIG. 5 shows a principle diagram of the present invention. 8 is a variable gain element,
9 is a switch, 10 is a level detection circuit, and 11 is a capacitor. From the connection in the figure, the gain of the variable gain element 8 changes according to the voltage of the capacitor 11. Time constant T ₁ for charging capacitor 11 at the signal level detected by level detection circuit 10
And the time constant T ₂ for discharging the capacitor 11 satisfies the relationship of T ₁ ≪T ₂ . That is, a so-called peak hold operation is performed. Further, the switch 9 is forcibly discharged by the switch 9 at the time of an empty time slot of the burst signal to reset the capacitor. Here, the guard time G or the like in FIG. 2 can be used as an empty time slot of the burst signal. This scheme takes advantage of the fact that the signal levels between each subscriber differ significantly, as shown in FIG. 2, but the signal levels (within one time slot) of one subscriber do not differ significantly. That is, the charging time constant T ₁ is set to a time within several bits of a burst (one time slot in terms of time), and the discharging time constant T ₂ is set to at least the maximum time during which “0” continues. That is, the configuration is such that the relationship of T ₁ ≪T ₂ is satisfied. Thus, within a single time slot, in response to short time of about several bits, and, since constant T ₂ when the discharge is set to more than at least one time slot, an input signal is "0" (here , “0”
Is described in association with the level low “L”), and a stable AGC operation is possible even for “1” (“H”) immediately after the continuation. The switch 9 forcibly discharges and resets the capacitor 11 by the switch 9 before the signal of the next time slot arrives at the level between each time slot which is greatly different. This makes it possible to achieve a stable AGC function that has a high-speed response and does not depend on the input signal pattern.

〔Example〕

FIG. 6 shows an example of a circuit configuration as a realizing means. 14
Is a high-speed operational amplifier (high-speed OP-AMP), 12 is an FET, 13 is a switch FET, and 15 is a diode. The gain is determined by the ratio between the feedback resistance _Rf of the amplifier 14 and the resistance between the source and drain of the FET 12. The resistance between the source and the drain of the FET 12 changes according to the gate voltage, and a variable gain element is constituted by the FET 12, the operational amplifier 14 used as the inverting amplifier, and the feedback resistor _Rf . The FET 13 is a switch, and is turned on / off by applying a control signal to the gate. In the level detection, peak detection is performed by the diode 15. The switch 13 is turned on in an empty time slot before the level is detected, and the capacitor 11 is forcibly discharged and reset. Before the burst signal of the new timeslot is input, switch
13 applies a control signal to turn off. Capacitor
Since the signal 11 is discharged, when a burst signal of a new time slot is input, the level is detected by the diode 15 while maintaining the maximum gain, and the capacitor 11 is detected by the charging time constant T ₁ determined by the forward resistance of the diode 15 and the capacitor 11. Charged. The gain decreases until the signal output reaches the forward voltage of the diode 15 and becomes a constant value. The time constant T ₂ for discharging the capacitor 11 is determined by the reverse resistance of the diode 15 and the capacitor 11.
Is determined by the capacitance value. Capacitor 11 with leakage current of FET12,13
Affect T ₂ charges of slightly discharged to but, T ₂ is substantially determined by the capacitance value of the reverse resistance and the capacitor 11 of the diode 15. Since the forward resistance and the reverse resistance of the diode 15 are significantly different, the relationship of T ₁ ≪T ₂ is satisfied.

Actually, an experiment was carried out with the circuit of FIG. 6 at a bit rate of 30 Mb / s, the length of each time slot was 125 μs, and the time slot interval was 2 μs, and the effect of the present invention was examined.
As a result, even if the signal level between the time slots differs by 20 dB, the first two bits of the time slot (about 60 ns)
Confirmed that AGC works.

FIG. 7 shows another example of the circuit configuration. 17 is a divider, 18 is an amplifier, and 16 is a switching transistor. The level detection method is the same as in the embodiment of FIG. Switch 16 FE
The operation is the same as in FIG. 6, except that T is changed to a transistor. An FET can be used as the switch element 16.
6 in that the variable gain element is realized by the divider 17. The divider 17 has a function of (input signal) ÷ (detection level signal) and has a constant output level.

The same effect can be obtained by using a multiplier 19 in place of the divider 17 as shown in FIG. Here, 19 is a multiplier, and 20 is a bias voltage generator.

In the circuits shown in FIGS. 6, 7 and 8, the switch control signal can be obtained from the signal by providing a signal idle time detection circuit, or can be obtained from the burst frame timing information. Is also possible.

(5) Effects of the Invention As described above, the present invention is composed of a variable gain element, a level detection circuit, a capacitor, and a switch, and controls the charge / discharge time constant of the capacitor, so that a signal in high-speed burst transmission is obtained. A high-speed AGC method for making the signal levels of bursts having different levels constant can be realized. This method can be implemented economically because it can be realized with simple circuit elements as shown in the embodiment.

[Brief description of the drawings]

FIG. 1 is a connection system diagram for explaining an optical communication system to which the system of the present invention can be applied, and FIG. 2 is a signal time of each subscriber when communicating using the optical communication system of FIG. FIG. 3 is a time chart illustrating an AGC operation, FIG. 4 is a block diagram illustrating a conventional AGC method, and FIG. 5 is a diagram illustrating a principle of the present invention. FIG. 6 is a block diagram for explaining an embodiment of the present invention, and FIGS. 7 and 8 are block diagrams for explaining another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1 ... Optical fiber transmission line, 2 ... Optical transmitter, 3 ... Optical receiver, 4 ... Star coupler, 5 ... Variable gain amplifier, 6
...... Low-pass filter, 7 ... Rectifier, 8 ... Gain variable element, 9 ... Switch, 10 ... Level detection circuit, 11 ... Capacitor, 12 ... FET, 13 ... Switch FET, 14 ... High-speed operational amplifier, 15: Diode, 16: Switching transistor, 17: Divider, 18: Amplifier, 19: Multiplier, 20: Bias voltage generator.

Continuation of the front page (56) References JP-A-61-194420 (JP, A) JP-A-60-81930 (JP, A) JP-A-60-152137 (JP, A) JP-A-1-200710 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H03G 3/20-3/34

Claims (2)

(57) [Claims] An AGC circuit for maintaining constant signal levels of bursts having different signal levels in high-speed burst transmission, comprising: a variable gain element, a level detection circuit, a capacitor, and a switch, wherein the variable gain element is When charging the capacitor at a signal level detected by the level detection circuit connected so that the gain changes according to the terminal voltage of the capacitor and further arranged to detect the output signal level of the variable gain element; constant T ₂ time for discharging the capacitor and constant T ₁ is, T ₁ << T satisfy the _second relation, and using said control signal derived from the frame timing information of the burst during the idle time slots of the burst A high-speed AGC circuit configured to forcibly discharge and reset the capacitor by a switch. 2. An AGC circuit for stabilizing signal levels of bursts having different signal levels in high-speed burst transmission, comprising: a variable gain element, a level detection circuit, a capacitor, and a switch, wherein the variable gain element is When charging the capacitor at a signal level detected by the level detection circuit connected so that the gain changes according to the terminal voltage of the capacitor and further arranged to detect the output signal level of the variable gain element; constant T ₂ time for discharging the capacitor and constant T ₁ is, T ₁ << T satisfy the _second relation, and using said control signal derived from the frame timing information of the burst when the burst guard time slot A high-speed AGC circuit configured to forcibly discharge and reset the capacitor by a switch.